Aluminum alloys, due to their light weight, have been used for casting engine blocks for internal combustion engines. Hypereutectic aluminum silicon alloys containing from 16% to 19% by weight of silicon are known to possess good wear resistant properties, achieved by the precipitated silicon crystals which constitute the primary phase.
U.S. Pat. No. 4,603,665 describes an improved hypereutectic aluminum silicon casting alloy having particular use in casting engine blocks for marine engines. The alloy of the aforementioned patent contains by weight from 16% to 19% silicon, up to 1.4% iron, 0.4% to 0.7% magnesium, up to 0.3% manganese, up to 0.37% copper, and the balance aluminum. By minimizing the copper content in the alloy, the ternary aluminum-silicon-copper eutectic is avoided and the resulting alloy has a relatively narrow solidification temperature range.
The alloy of U.S. Pat. No. 4,603,665, with a copper level below 0.37% by weight, solidifies over a temperature range about 45% less than the temperature range for the alloy with 4.5% copper and also shows significant improvement in microporosity in the solidified structure. However, with the use of the alloy of U.S. Pat. No. 4,603,665, in sand casting of large items, such as engine blocks, there is significant floatation of primary silicon particles into risers, resulting in a non-uniform distribution of primary silicon in the cast engine block. As the precipitated silicon is primarily responsible for the wear resistance of the alloy, a non-uniform distribution of primary silicon will adversely affect the wear resistance of the alloy.
Evaporable foam casting is a known technique in which a pattern formed of an evaporable foam material is supported in a mold and surrounded by an unbonded particulate medium, such as sand. When the molten metal contacts the pattern, the foam material vaporizes, with the vapor passing into the interstices of the sand, while the molten metal replaces the void formed by the vaporized foam material.
In an evaporable foam casting process, it is desirable to slow the solidification rate of the molten metal to provide time for the elimination of vapors generated by the decomposition of the pattern from the molten alloy. If the molten metal solidifies too swiftly, decomposition producers of the foam material can be entrapped in the metal, resulting in porosity and a loss of mechanical properties.